CHARACTERIZATION OF PHOSPHORUS SEGREGATION IN NEUTRON-IRRADIATED RUSSIAN PRESSURE-VESSEL STEEL WELD

An atom probe field ion microscopy characterization of three Russian p ressure vessel steels has been performed. Field ion micrographs of sev eral lath boundaries have indicated that they are decorated with a sem icontinuous film of discrete brightly-imaging precipitates that were i dentified as molybdenum carbonitrides. In addition, extremely high pho sphorus levels were measured at the lath boundaries. The phosphorus wa s found to be confined to an extremely narrow region indicative of mon olayer type segregation. The phosphorus coverage determined from the a tom probe results of the unirradiated materials agree with predictions based on McLean's equilibrium model of grain boundary segregation. Th e boundary phosphorus coverage of a neutron-irradiated weld material w as significantly higher than in the unirradiated material. Ultrafine d arkly-imaging copper- and phosphorus-enriched precipitates were also o bserved in the matrix of the neutron-irradiated material.